The lipids are a heterogenous group of compounds related to fatty acids. They are insoluble in water but soluble n organic solvents such as ether, alcohol, chloroform and benzene. Lipids include fats, oils, waxes, cholesterol, and related compounds.
Lipids as hydrophobic compounds, are components of cellular membranes, lipids are also used to store energy. Because of higher proportion of C-H bonds and very low proportion of oxygen, lipids store double the amount of energy as compared to the same amount of any carbohydrate. Some lipids provide insulation against atmospheric heat and cold also act as water proof material. Waxes, in the exoskeleton of insects, and cutin, an additional protective layer on the cuticle of epidermis of some plant organs e.g., leave fruits; seeds etc., and are some of the main examples.
Lipids have been classified as acylglycerols, waxes, phospholipids, sphinglipids, glycolipids and terpenoid lipids including carotenoids and steroids. The structure of some of these lipids is given below.
Acylglycerols
Acylglycerols are composed of glycerol and fatty acids (fig. 2.7). The most widely spread acyl glycerol is triacyl glycerol, also called triglycerides or neutral lipids. Chemically, acylglycerols can be defined as esters of fatty acids and alcohol. An ester is the compound produced as the result of a chemical reaction of an alcohol with an acid and a water molecule is released as shown below:
As indicated by dotted squares, OH is released from alcohol and H from an acid, H and OH combine and form a water molecule. Fatty acids are one of the most important components of triglycerides.
Fig. 2. 7: Triacylglycerol is composed of one glycerol and three fatty acids molecules.
Fatty acids contain even numbers (2 -30) of carbon atoms in straight chain attached with hydrogen and having an acidic group COOH (carboxylic group). They may contain no double bond (saturated fatty acids) or up to 6 double bonds (unsaturated fatty acids). In animals the fatty acids are straight chains (fig. 2.8), while in plants these may be branched or ringed. Solubility of fatty acids in organic solvents and their melting points increase with increasing number of carbon atoms in chain. Palmitic acid (C16) is much more soluble in organic solvent than butyric acid (C4). The melting point of palmitic acid is 63.1°C as against -8°C for butyric acid.
Fig. 2.8: some fatty acids with carbon numbers 2-18 are shown. Oleic acid is an unsaturated fatty acid (note a double bond between C9 and C10). Other fatty acids are saturated.
Fats containing unsaturated fatty acids are usually liquid at room temperature and are said to be oils. Fats containing saturated fatty acids are solids. Animal fats are solid at room temperature., whereas most of the plant fats are liquids. Fats and oils are lighter than water and have a specific gravity of about 0.8. They are not crystalline but some can be crystallized under specific conditions.
Waxes
Waxes are widespread as protective coatings on fruits and leaves. Some insects also secrets wax. Chemically, waxes are mixtures of long chain alkenes (with odd number of carbons ranging from C25 to C35) and alcohols, ketones and esters of long chain fatty acids. Waxes protect plants from water loss and abrasive damage. They also provide water barrier for insects, birds and animals such as sheep.
Phospholipids
Phospholipids are derivatives of phosphatidic acid (fig. 2.9), which are composed of glycerol, fatty acids and phosphoric acid, nitrogenous bases such as choline, ethanolamine and serine are important components of phospholipids. They are widespread in bacteria. Animal and plant cells and are frequently associated with membranes. Phosphatidylcholine is one of the common phospholipids.
Fig. 2.9: phosphatidic acid is composed of glycerol, 2 fatty acids (on C1 and C2), and a hosphoric acid on C3 of glycerol. In phospholipid a nitrogenous base (e.g. choline) is attached to phosphoric acid in phosphatidic acid.
Terpenoids
Terpenoids are a very large and important group of compounds which are made up of simple repeating units, isoprenoid units. This unit by condensation in different ways gives rise to compounds such as rubber, carotenoids, steroids terpenes etc.
Lipids constitute major source of energy, and play an important role in the structure of membranes of the cell and of organelles found in the cell. They also provide insulation, mechanical protection and protection from water loss and abrasive damage.